1. Field of the Invention
The present invention relates to a scanning apparatus for scanning a surface with a plural number of beam spots modulated independently of each other.
2. Description of Prior Art
In recent years there have been proposed various apparatus, for recording or reading data, that employs a light beam. In many of these known apparatus, the beam of light is deflected by a deflector and a surface to be scanned is scanned by a beam spot formed from the deflected beam. To increase the scanning efficiency of the scanning apparatus, it is also known to scan the surface with a plural number of beams at the same time. For such type of scanning apparatus using a plural number of beams it is often required to change the projection magnification with which the light beams are projected on the suface to be scanned. For example, if data are recorded on a photosensitive recording medium by such scanning apparatus, it is often required to change the size of the character to be recorded. An example of such scanning apparatus capable of changing projection magnification of light beams is disclosed in Japanese Patent Application Laid-Open No. Sho54-94053. This prior art scanning apparatus is shown in FIGS. 1A and 1B for the sake of reference in which FIG. 1A is a plan view thereof and FIG. 1B is a side development of the same.
In FIGS. 1A and 1B, reference numeral 1 designates a laser light source and 2 is a ultrasonic element for modulating and deflecting the incident light. 3 is a variable beam reforming lens system and 4 is a rotary polygonal mirror. The rotation axis of the polygonal mirror 4 extends in the direction normal to the plane of the paper, that is, in the direction parallel to Y-axis. 5 is a condenser lens for condensing the beams of light reflected by the polygonal mirror 4. 6 is a surface on which the beam is to be projected. 7 designates a high frequency driving electrical signal for the photo modulating element 2. The electrical signal contains different frequency components.
The beam of light emitted from the laser light source 1 is directed to the photo modulating element 2 driven by the driving electrical signal. In the photo modulating element 2, the beam is divided into a plural number (N) of beams in the Y-Z plane and the divided beam passes through the magnification variable beam reforming lens system 3. Thereafter, the beams are deflected by the rotary polygonal mirror 4 toward the condenser lens 5 through which the beams of light are condensed on the projection plane 6.
In the above system, change of magnification is attained by changing the scanning line space and the scanning line direction by changing the projection magnification of the magnification variable beam reforming lens system 3 and the input signal to the modulation element 2 respectively. However, this prior art scanning system has a drawback.
In the above system, the beam expansion in the direction normal to the deflection scan plane, that is, the beam expansion in the direction of the Y axis of the beams incident on the reflecting surfaces of the polygonal mirror becomes larger with increasing the projection magnification. It requires a large polygonal mirror. This is contrary to the desire for high speed rotation of polygonal mirror and therefore contrary to the desire for speed-up of scanning. Also, in the scanning system, a photo modulating element 2 is used to obtain a plural number of light beams. If there is used, as the light source part 1, a light source in the form of array such as semiconductor laser array or light emitting diode array which have been developed rapidly in recent years, then the expansion of the incident beams on the deflection reflecting surface of the polygonal mirror becomes larger also in the direction of deflection scan plane, that is, in X-Z plane. This also requires a larger rotary polygonal mirror which is contrary to the desire for high speed of scanning as described above.